This invention rlates to the purification, before discharge into the atmosphere, of gaseous effluents containing as impurities, oxides of sulphur, particularly sulphur dioxide. The toxic and pollutant properties of sulphur oxides (and certain nitrogen oxides) are such that these gases must be prevented from escaping into the atmosphere. In addition, it is economically desirable to recover these gases in the form of acids, more particularly sulphur oxides in the form of sulphuric acid. Many processes for purifying gaseous effluents in the form of, for example, smoke or fumes containing oxides of sulphur are known. Thus, it has been proposed to treat the smoke and fumes from steam generating stations, which smoke and fumes contain sulphur dioxide, either by catalysts by what is known as the dry method using vanadium salts or by injecting into the smoke or fumes ammonia whereby after a number of intermediate stages ammonium sulphate is formed. It has also been proposed to oxidise by means of ozonized oxygen the sulphur dioxide contained in such gaseous effluents, and to desulphurize the smoke or fumes by washing them with sulphuric acid containing ozonized oxygen, but it is found that in practice the smoke or fumes remain substantially unpurified even if a considerable quantity of ozone is employed. Thus, a disadvantage of the known processes for the treatment of gaseous effluents, e.g. smoke or fumes, by means of ozone is that they function with poor yields so that their operating cost is incompatible with economic requirements due to the excessive time taken up by the process or to the excessive consumption of ozone.
It is also known that salts of the transition metals, such as iron and manganese, are effective catalysts for the oxidation of sulphur dioxide with oxygen to form sulphur trioxide. U.S. Pat. Nos. 2,926,999 and 3,574,562, in the names of G. Tarbutton and M. Kawahata respectively, describe processes in which fumes containing sulphur oxides are treated in the presence of a manganese salt. The sulphuric acid is recovered in dilute form or in the form of ammonium sulphate which are products of limited value.
In other known processes, treatment of gaseous effluents with ozonized oxygen is carried out in a single column or in a single reactor, the ozonized oxygen being diluted in the fumes and the oxygen and ozone partial pressures being relatively low so that the manganese salts, which are used in the treatment, oxidize fairly slowly.